An active isoprenoid pyrophosphatase has been found in brain tissue, acting in vitro and possibly in vivo. From preliminary evidence the enzyme appears different from the corresponding enzyme in liver. Since the diversion of mevalonic acid into prenyl alcohols and acids may represent a control point in the biosynthesis of central nervous system cholesterol, the enzyme(s) and mechanism involved in brain cholesterol synthesis from mevalonic acid, isopentenyl pyrophosphate, isopentenol and isopentenoic acid in brain tissue will be studied in vitro and in vivo, with particular attention paid to the role such conversion may have in brain cholesterol biosynthetic control. A study of the characteristics of the enzyme involved and attempts to purify the latter will also constitute part of the study.